The present invention is directed to the field of tuning systems including station scanning provisions.
Tuning systems for radio and television receivers with station scanning or signal seeking provisions which automatically locate and tune the receiver to stations which are capable of being received by the receiver are desirable in order to minimize the inconvenience to a user of manually searching for tunable stations. Scanning or signal seeking tuning systems are described, for example, in U.S. Pat. Nos. 3,845,394 entitled, "Broadcast Receiver" and issued in the name of O. Hamada on Oct. 29, 1974 and 3,931,579 entitled, "Digital Signal Seeking System," and issued in the name of Ma et al. on Jan. 6, 1976 and 3,936,753.
Not only should a tuning system of a receiver including scanning provisions locate and tune the receiver to stations which are capable of being tuned, but it should also be capable of preventing the receiver to be erroneousy tuned to undesirable signals present in the intermediate frequency signal processing circuitry of the receiver. For example, without the proper precautions a television receiver employing a tuning system including channel scanning provisions may be erroneously tuned to adjacent channel carriers, signals containing a relatively large noise component or spurious signals due to intermodulation distortion or insufficient image rejection in the tuning circuitry of the receiver.
Moreover, it is desirable that a tuning system including scanning provisions to be capable of not only locating and tuning the receiver to carriers having standard broadcast frequencies, but it should desirably also be capable of locating and tuning the receiver to carriers having nonstandard frequencies arbitrarily offset from respective standard frequencies while also rejecting undesired signals. Carriers having nonstandard frequencies translated in frequency from respective standard frequency broadcast carriers associated with the various television channels a viewer may select may be produced when standard frequency broadcast carriers are processed by a television distribution system. A television tuning system including a phase locked loop configuration for generating the nominal frequency local oscillator signal to tune a receiver to the standard frequency associated with a channel selected by a viewer and another phase locked loop configuration, selectively enabled in accordance with a predetermined tuning algorithm, for reducing the frequency deviation between a picture carrier having a frequency offset from the nominal picture carrier frequency (e.g., in the United States 45.75 MHz) and the nominal picture carrier frequency is described in United States patent application Ser. No. 662,096 entitled, "Dual Mode Frequency Synthesizer for a Television Tuning Apparatus", filed in the name of R. M. Rast on Feb. 27, 1976.